P753: DYSFUNCTIONAL BONE MARROW ENDOTHELIAL PROGENITOR CELLS ARE INVOLVED IN PATIENTS WITH MYELODYSPLASTIC SYNDROMES
Authors: T. Xing, Z.-S. Lyu, C.-W. Duan, H.-Y. Zhao, S.-Q. Tang, Q. Wen, Y.-Y. Zhang, M. Lv, Y. Wang, L.-P. Xu, X.-H. Zhang, X.-J. Huang, Y. Kong
Journal: HemaSphere (2022)

Background: Myelodysplastic syndromes(MDS) are a group of heterogeneous myeloid clonal disorders characterized by ineffective haematopoiesis and immune deregulation. Emerging evidence has shown important roles of the bone marrow(BM) microenvironment in regulating haematopoiesis and immune balance. As an important component of BM microenvironment, murine studies and our previous studies(Blood2016, EbioMedicine2020, CEI2021) reported endothelial progenitor cells(EPCs) modulate the physiology and regeneration of haematopoietic stem cells(HSCs) and differentiation of effector T cell subsets. Some evidences suggest that EPCs demonstrate inferior supporting ability to normal HSCs in MDS. However, the dual supporting abilities of BM EPCs to normal HSCs and malignant cells in patients with different stages of MDS remain unclear. In addition, the immunomodulatory ability of BM EPCs in MDS needs to be elucidated. Aims: To determine the number, the functions and the abilities of haematopoiesis and immune regulation of BM EPCs in patients with different stages of MDS. Moreover, to explore the correlation between dysfunctional BM EPCs and different stages of MDS and underlying mechanisms of dysfunction. Methods: In this study, patients with lower-risk MDS(N=15), higher-risk MDS(N=15) and de novo acute myeloid leukaemia(AML)(N=15) and healthy donors(HDs)(N=15) were enrolled. The number of primary BM EPCs were detected by flow cytometry. The functions of BM EPCs were evaluated by DiI-Acetylated low-density lipoprotein uptake and FITC-UEA-1 binding assay, tube formation and migration assays. In order to assess the gene expression profiles of BM EPCs, RNA sequencing and gene set enrichment analysis were performed. The supporting abilities of BM EPCs from MDS patients to normal HSCs, leukaemia cells and T cells were assessed by in vitro coculture experiments(coculture with HSCs and T cells from HDs and HL-60 cells, respectively). Results: Increased but dysfunctional BM EPCs were found in MDS patients compared with HDs, especially in patients with higher-risk MDS. RNA-seq indicated the changes of haematopoiesis- and immune-related pathways in MDS BM EPCs. In vitro coculture experiments verified that BM EPCs from HD, lower-risk MDS, and higher-risk MDS to AML exhibited a progressively decreased ability to support normal HSCs, manifested as elevated apoptosis rates and intracellular reactive oxygen species(ROS) levels and decreased colony-forming unit plating efficiencies of HSCs. Moreover, BM EPCs from higher-risk MDS patients demonstrated an increased ability to support leukaemia cells, characterized by increased proliferation, leukaemia colony-forming unit plating efficiencies, whereas decreased apoptosis rates and apoptosis-related genes of HL-60 cells. Furthermore, BM EPCs induced normal T cell differentiation towards more immune-tolerant cells, like regulatory T cells, in higher-risk MDS patients in vitro. In addition, the levels of intracellular ROS and the apoptosis ratio were increased in BM EPCs from MDS patients than HDs, especially in higher-risk MDS patients, which may be underlying mechanisms of dysfunctional BM EPCs. Summary/Conclusion: The current study demonstrated that increased but dysfunctional BM EPCs in MDS patients. Moreover, the dysfunctions of BM EPCs were more severe in higher-risk than lower-risk MDS patients, manifested as more supportive ability to leukaemia cells but worse ability to normal HSCs. Our data indicated that repair of dysfunctional BM EPCs may be a promising therapeutic approach for MDS patients.